I replaced the subclassing example by one using a DataExtension. This
is the preferred way to add extra functionality to the Image class. You
can now add any existing image from the assets section instead of only
those belonging to the subclass.
Added a warning not to use has_many relations, because the UploadField
doesn't fully support them.
Documentation for the uploadfield. Only the implementation in the CMS
is documented, as the field doesn't seem to fully support being used in
the frontend just yet.